1. Field of the Invention
The present invention relates to an ignition device for an internal combustion engine which realizes downsizing and high reliability by reducing a power consumption with the use of an over-current protection circuit or an energization time abnormality protection circuit.
2. Description of the Related Art
There are ignition devices having a switching element and a variety of functions for protecting the switching element. For example, there are a breaker circuit for protection against a continuous energization state which is caused in the case where an ignition signal becomes a given value or more or in the case where a GND potential of a control device that generates the ignition signal becomes higher than the GND potential of an ignition power supply, a current limit circuit for protection against an over-current of a switching element which occurs due to the abnormality of a power supply voltage, and so on.
A conventional ignition device for an internal combustion engine will be described with reference to the accompanying drawings. FIG. 12 is a diagram showing the structure of the conventional ignition device for an internal combustion engine.
Referring to FIG. 12, reference numeral 1 denotes a control circuit, reference numeral 2 denotes a battery, reference numeral 3 denotes an ignition coil, reference numeral 4 denotes an ignition plug, and reference numeral 30 denotes a switching circuit.
Also, in the figure, the ignition device is made up of the ignition coil 3 and the switching circuit 30 including a switching element 5 that allows and interrupts the supply of a primary current to the ignition coil 3 on the basis of an ignition signal a from the control circuit 1.
The switching circuit 30 is made up of a wave form shaping circuit 8 that waveform-shapes the ignition signal a, the switching element 5, a protection circuit 6 for protecting the switching element 5 in the case where abnormality occurs in the ignition signal a, and a protection circuit 20 that turns off the switching element 5 at the time of abnormality of a power supply voltage.
The protection circuit 6 is an over-current protection circuit for protecting the switch element 5 against the over-current that occurs in the case where the ignition signal a becomes a given value or more, or in the case where the power supply voltage is abnormal. The over-current protection circuit 6 controls the input of the switching element 5 on the basis of a detection resistor 61 which is inserted in series to the primary coil of the ignition coil 3 and the switching element 5 and a result of comparing the terminal voltage with a given voltage of the reference power supply 62 so as to prevent a current of a given value or more from flowing.
Also, the protection circuit 20 is an over-voltage breaking circuit that breaks the primary current by turning off the input of the switching element 5 in the case where the power supply voltage abnormally comes up due to load dump or the like. The circuit is one to protect the power switch because the over-current protection circuit 6 operates due to a rise of the power supply voltage, and the power consumption of the switching element 5 becomes large, resulting in a fear that the power switch is broken.
In the case where a power is not supplied to the switching circuit 30 from the battery 2 (in the case where no power supply terminal is provided), it is general that the same function is provided to the control circuit 1 with respect to a serge such as the load dump, and the ignition signal is interrupted for protection. In the case where this function is not provided in the control circuit 1, it is necessary that a chip size or a radiator plate is enlarged for protection.
Then, the operation of the conventional ignition device for an internal combustion engine will be described with reference to the accompanying drawings. FIG. 13 is a timing chart showing the operation of the conventional ignition device for an internal combustion engine.
The switching element 5 is driven in response to a waveform-shaped signal e on the basis of the ignition signal a so as to allow or interrupt the supply of a primary current f to the ignition coil 3.
When the ignition signal a is normal, the current f that flows on the primary side of the ignition coil 3 has a target value as shown in FIG. 13. The supply of the primary current f to the ignition coil 3 is allowed or interrupted by the switching element 5 to generate a high voltage on the secondary side of the ignition coil 3 and ignite by the ignition plug 4.
On the other hand, in the case where the ignition signal a becomes long for some abnormality, when the current f of the primary coil which depends on the power supply voltage reaches a current value which is set by the over-current protection circuit 6, control is made so that the current f does not have the value or more on the basis of the output of the over-current protection circuit 6.
To make the power supply voltage of an automobile high is an important object for realizing the assumption of an electric load which is predicted in the future to be necessary to establish the environmental technology and the IT technology from the viewpoints of the international and global scales.
When an increase in the electric load is taking into consideration, it is more preferable that the power supply is higher, but from the viewpoint of safety, a 42 V power supply (battery voltage 36 V) is proposed.
The higher power supply has many advantages in view of the performance but has many difficulties in view of ensuring the safety and protection of the parts.
Similarly, in the ignition switching circuit, when a conventional product is used as it is, there arises a problem in that sufficient safety cannot be ensured.
For example, as to the protecting function against the over-current due to the abnormality of the ignition signal, even if the current can be limited, a voltage that is applied to the switching element becomes high, and the power consumption at the time of current limit becomes very large. As a result, even if a function which is originally added to protect the power switch is effected, there is the possibility that the switching element is broken.
In the conventional protection circuit thus structured, sufficient protection cannot be conducted to the higher power supply. For that reason, it is necessary to review modifications such as the application of a large allowable power of the switching element and the enlargement of the radiator plate or another protection circuit.